2026.6.22.1 — slugify: German umlaut transliteration

🐛 Fix — readable entity IDs for rooms with umlauts

slugify replaced every non-ASCII char with _, producing ugly IDs like
binary_sensor.k_che_presence_soft (Küche) and g_stezimmer_presence_soft
(Gästezimmer). Now German umlauts/eszett are transliterated before the
ASCII strip: ä→ae, ö→oe, ü→ue, ß→ss.

• „Küche" → kueche
• „Gästezimmer" → gaestezimmer
• Rooms without umlauts unchanged.

Per-entity unique_id is entry_id-based, so existing installs are unaffected
until their entity_id is renamed in the entity registry.